Transparent films made from crystallizable thermoplastics with a thickness from 1 to 500 μm are well known.
These films are not thermoformable and do not comprise any UV stabilizers as light stabilizers. Neither the films nor the items or moldings produced from them are therefore suitable for outdoor applications. Even after a short period in outdoor applications, these films and moldings exhibit yellowing and impairment of mechanical properties due to photooxidative degradation by sunlight.
EP-A-0 620 245 describes films with improved heat resistance. These films comprise antioxidants suitable for scavenging free radicals formed in the film and for degrading any peroxide formed. However, that specification makes no proposal as to how the UV resistance of films of this type may be improved. Nor does that specification state whether these films are suitable for thermoforming processes.
It is an object of the present invention, therefore, to provide a transparent, thermoformable film with a thickness which is preferably from 10 to 500 μm and which, in addition to good orientability, good mechanical properties, and also good optical properties, in particular has high UV resistance and thermoformability.
High UV resistance means that the films are not damaged, or are damaged only to an extremely small degree, by sunlight or by other UV radiation, and therefore the films and moldings produced from them are suitable for outdoor applications and/or critical indoor applications. In particular after a number of years of outdoor use the films should not yellow or show embrittlement or surface-cracking, nor show any impairment of mechanical properties. High UV resistance therefore means that the film absorbs UV light and transmits light only when the visible range has been reached.
Examples of good optical properties are high light transmittance (>84%), high surface gloss (>120), extremely low haze (<20%), and low Yellowness Index (YI<10).
Examples of good mechanical properties are high modulus of elasticity (EMD>3200 N/mm2; ETD>3500 N/mm2), and also good values for tensile stress at break (in MD>100 N/mm2; in TD>130 N/mm2).
Good orientability includes the ability of the film during its production to give excellent orientation in both longitudinal and transverse directions, without break-offs. An example of adequate thermoformability is the ability of the film to be thermoformed on commercially available thermoforming machinery without uneconomic predrying, to give complex and large-surface-area moldings.
The film of the invention should also be recyclable, in particular without loss of optical or mechanical properties, so that it can also be used for short-lived products and in the construction of exhibition stands, for example.